Hepatitis C virus-cross-reactive TCR gene-modified T cells: a model for immunotherapy against diseases with genomic instability.

A major obstacle hindering the development of effective immunity against viral infections, their associated disease, and certain cancers is their inherent genomic instability. Accumulation of mutations can alter processing and presentation of antigens recognized by antibodies and T cells that can lead to immune escape variants. Use of an agent that can intrinsically combat rapidly mutating viral or cancer-associated antigens would be quite advantageous in developing effective immunity against such disease. We propose that T cells harboring cross-reactive TCRs could serve as a therapeutic agent in these instances. With the use of hepatitis C virus, known for its genomic instability as a model for mutated antigen recognition, we demonstrate cross-reactivity against immunogenic and mutagenic nonstructural protein 3:1406-1415 and nonstructural protein 3:1073-1081 epitopes in PBL-derived, TCR-gene-modified T cells. These single TCR-engineered T cells can CD8-independently recognize naturally occurring and epidemiologically relevant mutant variants. TCR-peptide MHC modeling data allow us to rationalize how TCR structural properties accommodate recognition of certain mutated epitopes and how these substitutions impact the requirement of CD8 affinity enhancement for recognition. A better understanding of such TCRs' promiscuous behavior may allow for exploitation of these properties to develop novel, adoptive T cell-based therapies for viral infections and cancers exhibiting similar genomic instability.

A coreceptor-independent transgenic human TCR mediates anti-tumor and anti-self immunity in mice.

Recent advancements in T cell immunotherapy suggest that T cells engineered with high-affinity TCR can offer better tumor regression. However, whether a high-affinity TCR alone is sufficient to control tumor growth, or the T cell subset bearing the TCR is also important remains unclear. Using the human tyrosinase epitope-reactive, CD8-independent, high-affinity TCR isolated from MHC class I-restricted CD4(+) T cells obtained from tumor-infiltrating lymphocytes (TIL) of a metastatic melanoma patient, we developed a novel TCR transgenic mouse with a C57BL/6 background. This HLA-A2-restricted TCR was positively selected on both CD4(+) and CD8(+) single-positive cells. However, when the TCR transgenic mouse was developed with a HLA-A2 background, the transgenic TCR was primarily expressed by CD3(+)CD4(-)CD8(-) double-negative T cells. TIL 1383I TCR transgenic CD4(+), CD8(+), and CD4(-)CD8(-) T cells were functional and retained the ability to control tumor growth without the need for vaccination or cytokine support in vivo. Furthermore, the HLA-A2(+)/human tyrosinase TCR double-transgenic mice developed spontaneous hair depigmentation and had visual defects that progressed with age. Our data show that the expression of the high-affinity TIL 1383I TCR alone in CD3(+) T cells is sufficient to control the growth of murine and human melanoma, and the presence or absence of CD4 and CD8 coreceptors had little effect on its functional capacity.

NKG2D signaling on CD8⁺ T cells represses T-bet and rescues CD4-unhelped CD8⁺ T cell memory recall but not effector responses.

CD4-unhelped CD8(+) T cells are functionally defective T cells primed in the absence of CD4(+) T cell help. Given the co-stimulatory role of natural-killer group 2, member D protein (NKG2D) on CD8(+) T cells, we investigated its ability to rescue these immunologically impotent cells. We demonstrate that augmented co-stimulation through NKG2D during priming paradoxically rescues memory, but not effector, CD8(+) T cell responses. NKG2D-mediated rescue is characterized by reversal of elevated transcription factor T-box expressed in T cells (T-bet) expression and recovery of interleukin-2 and interferon-γ production and cytolytic responses. Rescue is abrogated in CD8(+) T cells lacking NKG2D. Augmented co-stimulation through NKG2D confers a high rate of survival to mice lacking CD4(+) T cells in a CD4-dependent influenza model and rescues HIV-specific CD8(+) T cell responses from CD4-deficient HIV-positive donors. These findings demonstrate that augmented co-stimulation through NKG2D is effective in rescuing CD4-unhelped CD8(+) T cells from their pathophysiological fate and may provide therapeutic benefits.

CD8 co-receptor promotes susceptibility of CD8+ T cells to transforming growth factor-ß (TGF-ß)-mediated suppression.

CD8+ T cell function depends on a finely orchestrated balance of activation/suppression signals. While the stimulatory role of the CD8 co-receptor and pleiotropic capabilities of TGF-ß have been studied individually, the influence of CD8 co-receptor on TGF-ß function in CD8+ T cells is unknown. Here, we show that while CD8 enhances T cell activation, it also enhances susceptibility to TGF-ß-mediated immune suppression. Using Jurkat cells expressing a full-length, truncated or no αßCD8 molecule, we demonstrate that cells expressing full-length αßCD8 were highly susceptible, αßCD8-truncated cells were partially susceptible, and CD8-deficient cells were completely resistant to suppression by TGF-ß. Additionally, we determined that inhibition of Lck rendered mouse CD8+ T cells highly resistant to TGF-ß suppression. Resistance was not associated with TGF-ß receptor expression but did correlate with decreased Smad3 and increased Smad7 levels. These findings highlight a previously unrecognized third role for CD8 co-receptor which appears to prepare activated CD8+ T cells for response to TGF-ß. Based on the important role which TGF-ß-mediated suppression plays in tumor immunology, these findings unveil necessary considerations in formulation of CD8+ T cell-related cancer immunotherapy strategies.

Relationship between CD8-dependent antigen recognition, T cell functional avidity, and tumor cell recognition.

Effective immunotherapy using T cell receptor (TCR) gene-modified T cells requires an understanding of the relationship between TCR affinity and functional avidity of T cells. In this study, we evaluate the relative affinity of two TCRs isolated from HLA-A2-restricted, gp100-reactive T cell clones with extremely high functional avidity. Furthermore, one of these T cell clones, was CD4- CD8- indicating that antigen recognition by this clone was CD8 independent. However, when these TCRs were expressed in CD8- Jurkat cells, the resulting Jurkat cells recognized gp100:209-217 peptide loaded T2 cells and had high functional avidity, but could not recognize HLA-A2+ melanoma cells expressing gp100. Tumor cell recognition by Jurkat cells expressing these TCRs could not be induced by exogenously loading the tumor cells with the native gp100:209-217 peptide. These results indicate that functional avidity of a T cell does not necessarily correlate with TCR affinity and CD8-independent antigen recognition by a T cell does not always mean its TCR will transfer CD8-independence to other effector cells. The implications of these findings are that T cells can modulate their functional avidity independent of the affinity of their TCRs.

Influence of human CD8 on antigen recognition by T-cell receptor-transduced cells.

The CD8 coreceptor on T cells has two functions. Namely, CD8 acts to stabilize the binding of the T-cell receptor (TCR) to the peptide-MHC complex while localizing p56(lck) (lck) to the TCR/CD3 complex to facilitate early signaling events. Although both functions may be critical for efficient activation of a CTL, little is known about how the structural versus signaling roles of CD8, together with the relative strength of the TCR, influences T-cell function. We have addressed these issues by introducing full-length and truncated versions of the CD8alpha and CD8beta chains into CD8(-) Jurkat cell clones expressing cloned TCRs with known antigen specificity and relative affinities. Using a combination of antigen recognition and tetramer-binding assays, we find that the intracellular lck-binding domain of CD8 is critical for enhanced T-cell activation regardless of the relative strength of the TCR. In contrast, the extracellular domain of CD8 seems to be critical for TCRs with lower affinity but not those with higher affinity. Based on our results, we conclude that there are different requirements for CD8 to enhance T-cell function depending on the strength of its TCR.

Identification of a hepatitis C virus-reactive T cell receptor that does not require CD8 for target cell recognition.

Hepatitis C virus (HCV) has been reported to elicit B and T cell immunity in infected patients. Despite the presence of antiviral immunity, many patients develop chronic infections leading to cirrhosis, hepatocellular carcinoma, and liver failure that can require transplantation. We have previously described the presence of HLA-A2-restricted, HCV NS3-reactive cytotoxic T lymphocytes (CTL) in the blood of HLA-A2- liver transplantation patients that received an HLA-A2+ liver allograft. These T cells are analogous to the "allospecific" T cells that have been described in hematopoietic stem cell transplantation patients. It has been speculated that allospecific T cells express high-affinity T cell receptors (TCRs). To determine if our HCV-reactive T cells expressed TCRs with relatively high affinity for antigen, we identified and cloned a TCR from an allospecific HLA-A2-restricted, HCV:NS3:1406-1415-reactive CD8+ T cell clone and expressed this HCV TCR in Jurkat cells. Tetramer binding to HCV TCR-transduced Jurkat cells required CD8 expression, whereas antigen recognition did not. In conclusion, based on the reactivity of the TCR-transduced Jurkat cells, we have identified a TCR that transfers anti-HCV reactivity to alternate effectors. These data suggest this high affinity HCV-specific TCR might have potential new immunotherapic implications.

T-cell receptor tetramer binding or the lack there of does not necessitate antigen reactivity in T-cell receptor transduced T cells.

Genetic transfer of T-cell receptor (TCR) chains provides a means of transferring tumor antigen specificity onto an alternate T-cell population. To determine which tumor reactive TCRs are best suitable for such adoptive transfer, careful evaluation of the resulting TCR modified populations need to be performed. We have previously cloned, and expressed TCRs from melanoma, EBV, HCV, and HPV reactive T-cell clones and found that several routine indicators of T-cell function do not always predict the relative strength of a TCR. Using a combination of tetramer binding assays and antigen recognition assays, we identified TCRs that fall into three classes. One class of TCR did not bind tetramers yet resulted in cells with high avidity for antigen. A second TCR class bound tetramer but did not secrete cytokines in response to antigen. Finally, the third class of TCRs bound tetramer and reacted to antigen as would be expected. We conclude that tetramer binding is not always a good indicator of the function of a cloned TCR or the avidity of a TCR gene modified T cell. Furthermore, our data indicate that the use of tetramer binding alone to identify antigen reactive TCRs may result in the exclusion of TCRs that may be highly reactive or cross reactive to the relevant tumor antigen.

Simultaneous generation of CD8+ and CD4+ melanoma-reactive T cells by retroviral-mediated transfer of a single T-cell receptor.

Adoptive immunotherapy of cancer requires the generation of large numbers of tumor antigen-reactive T cells for transfer into cancer patients. Genes encoding tumor antigen-specific T-cell receptors can be introduced into primary human T cells by retroviral mediated gene transfer as a potential method of providing any patient with a source of autologous tumor-reactive T cells. A T-cell receptor-specific for a class I MHC (HLA-A2)-restricted epitope of the melanoma antigen tyrosinase was isolated from a CD4(+) tumor-infiltrating lymphocyte (TIL 1383I) and introduced into normal human peripheral blood lymphocytes by retroviral transduction. T-cell receptor-transduced T cells secreted various cytokines when cocultured with tyrosinase peptide-loaded antigen-presenting cells as well as melanoma cells in an HLA-A2-restricted manner, and could also lyse target cells. Furthermore, T-cell clones isolated from these cultures showed both CD8(+) and CD4(+) transduced T cells could recognize HLA-A2(+) melanoma cells, giving us the possibility of engineering class I MHC-restricted effector and T helper cells against melanoma. The ability to confer class I MHC-restricted tumor cell recognition to CD4(+) T cells makes the TIL 1383I TCR an attractive candidate for T-cell receptor gene transfer-based immunotherapy.